The present invention relates generally to an electric double layer capacitor, and more particularly to a high performance double layer capacitor made with low-resistance aluminum-impregnated carbon-cloth electrodes and a high performance electrolytic solution.
Double layer capacitors, also referred to as electrochemical capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, they can typically deliver the stored energy at a higher power rating than rechargeable batteries. Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the cell. Coupled to the back of each of the active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
Double layer capacitors store electrostatic energy in a polarized liquid layer which forms when a potential exists between two electrodes immersed in an electrolyte. When the potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name xe2x80x9cdouble layerxe2x80x9d capacitor) by the polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes.
The use of carbon electrodes in electrochemical capacitors with high power and energy density represents a significant advantage in this technology because carbon has a low density and carbon electrodes can be fabricated with very high surface areas. Fabrication of double layer capacitors with carbon electrodes has been known in the art for quite some time, as evidenced by U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the high contact resistance of the internal carbon-carbon contacts, and the contact resistance of the electrodes with a current collector. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC (resistance x capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
Various electrode fabrication techniques have been disclosed over recent years. For example, the Yoshida et al. patent (U.S. Pat. No. 5,150,283) discloses a method of connecting a carbon electrode to a current collector by depositing carbon powder and other electrical conductivity-improving agents on an aluminum substrate.
Another related approach for reducing the internal resistance of carbon electrodes is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.) which teaches that the incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished through weaving metallic fibers into carbon fiber preforms.
Yet another approach for reducing the resistance of a carbon electrode is taught in U.S. Pat. No. 4,562,511 (Nishino et al.) wherein the carbon fiber is dipped into an aqueous solution to form a layer of a conductive metal oxide, and preferably a transition metal oxide, in the pores of the carbon fibers. Nishino et al. also discloses the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition.
Still another related approach for achieving low resistance is disclosed in U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk et al.). The Tatarchuk et al. patents demonstrate that metal fibers can be intermixed with a carbon preform and sintered to create a structurally stable conductive matrix which may be used as an electrode. The Tatarchuk et al. patents also teach a process that reduces the electrical resistance in the electrode by reducing the number of carbon-carbon contacts through which current must flow to reach the metal conductor. This approach works well if stainless steel or nickel fibers are used as the metal. However, applicants have learned that this approach has not been successful when aluminum fibers are used because of the formation of aluminum carbide during the sintering or heating of the electrode.
Another area of concern in the fabrication of double layer capacitors relates to the method of connecting the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor, and such internal resistance must be kept as low as possible.
U.S. Pat. No. 4,562,511 (Nishino et al.) suggests plasma spraying of molten metals such as aluminum onto one side of a polarizable electrode to form a current collector layer on the surface of the electrode. Alternative techniques for bonding and/or forming the current collector are also considered in the ""511 Nishino et al. patent, including arc-spraying, vacuum deposition, sputtering, non-electrolytic plating, and use of conductive paints.
The previously-cited Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963) show the bonding of a metal foil current collector to the electrode by sinter bonding the metal foil to the electrode element.
U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method of bonding the current collector to the surface of the electrode by a hot curing process which causes the material of the current collectors to enter the pores of the electrode elements.
Still other related art concerned with the method of fabricating and adhering current collector plates can be found in U.S. Pat. Nos. 5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued to Kurabayashi et al.
It is thus apparent that there is a continuing need for improved double layer capacitors. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings within a relatively short period of time. Such improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is also apparent that improvements are needed in the techniques and methods of fabricating double layer capacitor electrodes so as to lower the internal resistance of the double layer capacitor and maximize the operating voltage. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. It is thus readily apparent that improved techniques and methods are needed to lower the internal resistance of the electrodes used within a double layer capacitor and increase the operating voltage.
The present invention addresses the above and other needs by providing a high performance double layer capacitor having multiple electrodes wherein the multiple electrodes are made from activated carbon that is volume impregnated with aluminum in order to significantly reduce the internal electrode resistance by decreasing the contact resistance between the activated carbon elements. More particularly, the high performance double layer capacitor of the present invention includes at least one pair of aluminum-impregnated carbon electrodes, each being formed by volume impregnating an activated carbon preform (i.e., a carbon cloth) with aluminum, or other suitable metal, e.g., titanium, with each electrode being separated from the other by a porous separator, and with both electrodes being saturated with a high performance electrolytic solution.
In accordance with one aspect of the invention, a high performance double layer capacitor is provided that is made as a single cell, multi-electrode capacitor. By xe2x80x9csingle cellxe2x80x9d, it is meant that only one electrolytic solution seal is required, even though multiple parallel-connected aluminum-impregnated carbon electrodes are utilized. Such a single cell multi-electrode double layer capacitor, in one embodiment, includes first and second flat stacks of composite electrodes adapted to be housed in a closeable two-part capacitor case. Advantageously, the case represents the only component of the capacitor that must be sealed to prevent electrolyte leakage. Each electrode stack has a plurality of aluminum-impregnated carbon electrodes connected in parallel, with the electrodes of one stack being interleaved with the electrodes of the other stack to form an interleaved stack, and with the electrodes of each stack being electrically connected to respective capacitor terminals. A porous separator is positioned against the electrodes of one stack before interleaving to prevent electrical shorts between the electrodes when they are interleaved. In an alternative embodiment, the electrodes and separator may be spirally wound rather than interleaved in flat stacks.
The electrodes are preferably made by folding a compressible, very low resistance, metal-impregnated carbon cloth (the cloth being made from activated carbon fibers) around a current collector foil. In the flat stack embodiment, the current collector foils of each respective stack are connected in parallel to each other and to the respective capacitor terminal. In the spirally wound embodiment, the current collector foil of each electrode is connected to the respective capacitor terminal. The preferred metal that is impregnated into the carbon cloth comprises aluminum, although other metals may also be used, e.g., titanium. For the flat stack embodiment, the height of the unconfined interleaved stack is by design somewhat greater than the inside height of the closed capacitor case, thereby requiring a slight compression of the interleaved electrode stack when placed inside of the case. This slight compression advantageously maintains the interleaved electrode stack under a modest constant pressure (e.g., 10 psi) while held inside of the case. In the spiral wound embodiment, the winding of the electrodes requires a slight radial compression in order to fit within the closed capacitor case. In either embodiment, the modest pressure helps assure a low contact resistance between the current collector foils and the aluminum-impregnated carbon cloth electrodes. The closed capacitor case is filled with an appropriate electrolytic solution and sealed. A preferred electrolytic solution is made by dissolving an selected salt into acetonitrile (CH3CN).
In accordance with another aspect of the invention, the two parts of the capacitor case may be conductive and insulated from each other when the capacitor case is assembled, thereby allowing each half of the case to function as the capacitor terminal.
One embodiment of a high performance double layer capacitor made as described herein exhibits a capacitance of about 2400 Farads, an energy density in the range of 2.9 to 3.5 W-hr/kg at an operating voltage of 2.3 volts, a power rating of about 1000 W/kg at a 400 ampere discharge, an electrode resistance of about 0.8 milliohms (mxcexa9), and a time constant of about 2 seconds. Such performance parameters, to applicants"" knowledge, represent a significant and remarkable advance over what has heretofore been available in the double layer capacitor art.
In accordance with yet another aspect of the invention, the flat stack capacitor design lends itself to multi-electrode scale up or scale down in order to meet the needs of a particular double layer capacitor application. Thus, by simply increasing or decreasing the size and number of composite electrodes that are used within the interleaved electrode stack, and by making appropriate scaled changes in the physical parameters (size, weight, volume) of the capacitor, it is possible to provide a high performance double layer capacitor that is tailored to a specific application. With such a capacitor, the door is thus opened to a wide variety of applications wherein relatively large amounts of energy must be stored and retrieved from a compact storage device in a relatively short period of time. Similar scaling is also readily achievable using the spiral-wound embodiment.
The present invention is further directed to improved methods of making a high performance double layer capacitor. Such methods include, e.g., impregnating molten aluminum into a commercially-available carbon cloth comprising a weave of bundles of activated carbon fibers. The transverse resistance of the carbon cloth is reduced dramatically, e.g., by a factor of fifty, by impregnating molten aluminum deep into the tow of the fiber bundles. The aluminum-impregnated carbon cloth serves as the key component of each electrode within the double layer capacitor. Electrical contact is made with the impregnated carbon cloth by way of a foil current collector which contacts the impregnated side of the cloth on both sides of the foil, i.e., the impregnated cloth is folded around the foil current collector so that both sides of the foil current collector contact the impregnated side of the folded cloth. The contact resistance between the foil current collector and the carbon cloth is reduced by applying pressure to the impregnated cloth prior to assembly within the capacitor to smooth out the hills and valleys at the impregnated surface, thereby increasing the surface area which contacts the foil current collector.
The large surface area provided by the carbon cloth of each composite electrode used with the invention may be multiplied many times by interleaving a large number of such composite electrodes, e.g., 54 electrodes. The interleaved aluminum-impregnated electrodes are separated by a suitable porous separator which provides electrical insulation between the electrodes, yet permits the ions of an electrolytic solution to readily pass therethrough. The foil current collectors of alternating electrodes, e.g., the foil current collectors of 27 of the electrodes, are electrically connected in parallel and connected to a suitable capacitor terminal. Similarly, the foil current collectors of the remaining electrodes are also electrically connected in parallel and connected to the other capacitor terminal. The interleaved stack of electrodes is then sealed in a suitable capacitor case, which case maintains the interleaved stack under a modest pressure to reduce the contact resistance. The inside of the case is then evacuated and dried, and filled with a highly conductive non-aqueous electrolytic solution made from a suitable solvent mixed with a specified salt.
Accordingly, it is a feature of the present invention to provide a high performance double layer capacitor, and method of making such capacitor, having a relatively high energy density of greater than about 3.4 W-hr/kg at an operating voltage of 2.3 volts.
It is another feature of the invention to provide an improved double layer capacitor having a maximum useable power density of greater than about 1000 W/kg.
It is a further feature of the invention to provide an improved double layer capacitor having a low internal resistance in combination with a high capacitance such that the characteristic RC time constant of the capacitor remains at a value which allows for relatively rapid charge/discharge times. For example, in one embodiment, the resistance of the capacitor is less than about 0.9 mxcexa9, while the capacitance is at least 2350 Farads, thereby allowing the charging and discharging of the capacitor (into a zero impedance load, or short) to occur at a time constant of about 2 seconds.
Another important feature of the invention is the identified use of advanced non-aqueous electrolytic solutions that allow higher operating voltages of the capacitor. A preferred electrolytic solution is, for example, mixed using an acetonitrile (CH3CN) solvent, and a suitable salt, which electrolyte allows a nominal operating voltage of 2.3 volts, with peak voltages of up to 3.0 volts or higher.